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  <title>Document</title>
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<body>
  <script>
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    function flatten(arr) {
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      console.log(arr)
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    console.log(flatten(arr));


    let arrto = [
      {
        id: 1,
        children: [
          {
            id: 11,
            children: [
              { id: 111, }
            ]
          },
          { id: 12, }
        ]
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      {
        id: 2,
        children: [
          {
            id: 21,
            children: []

          }
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      }
    ]

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      return arrto.reduce((result, item) => {
        return result.concat(
          item,
          Array.isArray(item.children) ? flatten(item.children) : []
        );
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    }
    console.log(flatten(arrto));

  </script>
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